1. The effects of swim-induced stress on rat atrial norepinephrine content, plasma corticosterone level and pacemaker sensitivity to catecholamines were studied. 2. Swimming for 50 min reduced the atrial norepinephrine content and induced a long-lasting increase in the plasma corticosterone level. 3. Pacemakers isolated from swim stressed rats showed supersensitivity to the chronotropic effect of isoprenaline. Sensitivity to norepinephrine was not significantly altered. Bilateral adrenalectomy or metyrapone pretreatment prevented the development of swimming-induced supersensitivity to isoprenaline. After in vitro denervation and addition of cocaine supersensitivity to epinephrine was demonstrated. 4. It is concluded that corticosterone, at least partially, mediates the supersensitivity to isoprenaline and epinephrine in pacemaker isolated from acutely swim stressed rats.19129-35